I was motivated to provide a simple inexpensive liquid level switch apparatus for producing an output signal, when the liquid level being monitored is at a high level in a tank, which can shut down a valve halting further liquid to be supplied to the tank to prevent overflow. I also deemed it desirable to have the apparatus produce an output signal when the liquid level being monitored is at a low level in a tank that can cause a valve to supply further liquid to initiate or increase the flow rate of liquid being admitted into the tank.
Such apparatus may also advantageously produce an intermediate level signal as the liquid level continues to fall toward a lower level range. For example, as fuel oil is being consumed in heating a home, a warning indicator can be produced to continuously signal that home owner should make arrangements to have the fuel oil delivered. If the fuel oil is just about to run out, a low level detector can produce an urgency signal when a particular low liquid level is detected.